The present invention relates to a method for firing ceramic cores for use in the casting of molten metallic materials.
Most manufacturers of gas turbine engines are evaluating advanced investment cast turbine airfoils (i.e. turbine blade or vane) which include intricate air cooling channels to improve efficiency of airfoil internal cooling to permit greater engine thrust and provide satisfactory airfoil service life. Internal cooling passages are formed in the cast airfoils using one or more thin airfoil shaped ceramic cores positioned in a ceramic shell mold where the molten metal is cast in the mold about the core. After the molten metal solidifies, the mold and core are removed to leave a cast airfoil with one or more internal passages where the cores formerly resided.
The ceramic core is typically made using a plasticized ceramic compound comprising ceramic flour, binder and various additives. The ceramic compound is injection molded, transfer molded or poured at elevated temperature in a core die or mold. When the green (unfired) core is removed from the die or mold, it typically is placed between top and bottom setters to cool to ambient temperature before core finishing and gauging operations and firing at an elevated sintering temperature. The green ceramic core is fired at elevated (superambient) temperature in one or more steps to sinter and strengthen the core for use in casting metallic material, such as a nickel base or cobalt base superalloy. U.S. Pat. No. 5,014,763 discloses a green ceramic core positioned top and bottom setters during firing.
The green core can exhibit distortion from stresses induced in the core from the molding and/or ambient cooling operations. Distortion can be a particular problem with respect to the airfoil region of the core have a trailing edge with a relatively thin cross-section that is prone to distortion. As a result, the green ceramic cores can exhibit dimensional variations from one core to the next in a production run of cores. Moreover, the green core may be improperly contacted by the top or bottom setter such that dimensional variations from one core to the next occur in a production run.
An object of the present invention is to provide a method of firing ceramic cores in a manner to reduce dimensional variations and improve yield of ceramic cores that meet dimensional tolerances.
In an illustrative embodiment of the invention, a method for firing a ceramic core comprises placing an unfired (green) or previously fired ceramic core having a molded core shape on a setter, placing at least one flexible refractory bag containing refractory particulates on the ceramic core to apply a force on the core toward the setter during firing, and then heating the ceramic core on the setter to an elevated superambient firing temperature to sinter and strengthen the core for use in casting metallic material. Heating of the ceramic core with the flexible weight bag(s) thereon pursuant to the invention helps conform the core to a surface of the setter to reduce dimensional distortion of the core and improve yields of cores within preselected dimensional tolerances. Preferably, a plurality of the flexible refractory bags of selected weight are placed on the core at orientations and locations found effective to reduce dimensional variation of the core from print dimensions. The ceramic core can be treated pursuant to the invention in the unfired (green) condition or in a fired condition.
In a particular embodiment of the invention, the flexible bag comprises a ceramic fiber cloth bag that contains a ceramic grog (e.g. ceramic particulates). The bag prevents spreading of the grog particulates during firing and prevents chemical reaction between the grog particulates and the core. The bag can be in the configuration of a tubular sock having opposite ends that are closed after the sock is filled with the grog by a respective metallic wire or ceramic string that can withstand the temperature of firing.
The invention is beneficial for, although not limited to, firing a relatively large ceramic core that includes an airfoil region that is prone to distortion. For example only, the invention is beneficial for firing ceramic cores used in the investment casting of airfoils (e.g. turbine blades and vanes) of land-based and aero gas turbine engines.